The objectives of this project are to: (1) identify the types and functions of mononuclear cells infiltrating the tumor mass at different stages of growth, i.e., from the time of tumor take through rapid growth and spontaneous regression; (2) characterize the mechanism responsible for spontaneous regression of tumors; and (3) determine the mechanism of maternal-offspring transfer of tumor immunity. The studies of the past year showed that the majority of mononuclear cells infiltrating the tumors, especially at the time of spontaneous regression, were TG lymphocytes. Competitive ELISA assay of the tumor-associated antigen in the sera of tumor-bearing animals showed that the antigen levels were a quadratic function of the tumor volumes, suggesting a role of antigen in progressive growth of the tumor.